Reflective targets are widely used in surveying applications, such as in construction or surface mining. A reflective target may be applied to a landmark or position of interest and aimed at using a surveying instrument, such as a theodolite, having a light emitting element for emitting light such as laser light along an optical axis towards the reflective target. The surveying instrument includes an optical arrangement for receiving the reflected light from the reflective target. Based on the reflected light the optical axis of the surveying instrument may be accurately oriented towards the target. Angle measurements with respect to the target may be conducted using horizontal and vertical angle scales of the surveying instrument.
Further, the reflective target may be tracked if the target is attached to a moving object such as a machine on a construction site or other vehicle. For tracking the optical axis of the surveying instrument is continuously oriented towards the reflective target and thus follows the reflective target along its path using motorized units for adjusting the horizontal and vertical inclination of the surveying instrument.
Reflective targets usually include prisms that reflect the incident light exactly in the direction of incidence. An example of a prism is shown in FIG. 10, generally denoted by reference numeral 900. The prism 900 is constituted by a section of a corner cube with its rear side defined by 3 perpendicular surfaces. Where the rear surfaces abut at each other they form three mutually perpendicular edges 910 on the rear side of the prism. The front surface of the prism, illustrated by the hatched surface denoted 920, has a hexagonal shape. Incident light entering the prism through the front surface 920 within a certain range of angles of incidence is refracted by the front surface 920 and then reflected three times, once at each of the three perpendicular surfaces. The reflected light is then again refracted at the front surface 920 and leaves the prism through the front surface 920 in exactly the same direction as it entered the prism.
The rear surfaces of the prism may be uncoated or coated with a reflective layer. If the prism is uncoated best reflection at each of the 3 perpendicular surfaces occurs if the condition for total reflection is given. If the angle of incidence of the light beam on one of the 3 perpendicular surfaces falls below the critical angle of total reflection only weak reflection occurs due to Fresnel effects. To guarantee total reflection at the 3 perpendicular surfaces the orientation of the prism may advantageously be used with following ranges of angle of incidence of the light on the front surface.
If the prism is oriented as shown in FIG. 10, with the front surface in the drawing plane, the angle range around the direction of perpendicular incidence of the light into the drawing plane, i.e. normal to the drawing plane, is approximately 20° to the left and 60° to the right, depending on the refractive index, as illustrated by the corresponding arrows in FIG. 10. Further, the useful angle range of incident light around perpendicular incidence is approximately 23° toward the top and 23° towards the bottom of the page, depending on the refractive index, as illustrated in FIG. 10.
FIG. 11 illustrates a photographic top view of a prism 1000. In FIG. 11 the perpendicular edges formed by the rear surfaces of the prism are denoted by reference numeral 1010. Reference numerals 1011 denote reflections of the edges of 1010 that do not have any physical representation on the side of the corner cube. The reflections of the perpendicular edges 1010 are visible when the prism 1000 is viewed from the front side.
High precision prisms generally are made of glass. Low-cost prisms usually are made of molded plastic, with a plurality of prisms arranged adjacent to one another, such as shown in FIG. 12. Arrangements of molded plastic prisms such as shown in FIG. 12 with a plurality of prisms such as prisms 900 of FIG. 10, generally are used e.g. as reflectors for vehicles and light barriers. However, the reflector shown in FIG. 12 is planar, and if the angle of incidence of the light is outside the ranges as illustrated in FIG. 10, the reflector cannot be used for measurements in surveying applications.